This invention relates to radio antenna apparatus and it particularly relates to an antenna configuration which is useful for providing multiple band and frequency coverage while employing a single feedline to supply and receive radio frequency energy.
One of the more difficult problems in radio antenna construction and performance is that of achieving a high degree of efficiency while maintaining a correct impedance match across a wide range of frequencies. An antenna element typically exhibits resonance wherein it absorbs energy from a source such as a transmission line and transmitter more readily at some frequencies than at others.
It is often desirable to have communications capability at various frequencies throughout the high frequency spectrum. A high frequency band antenna should be effective on as many frequencies as possible. Resonance behavior inhibits effective impedance matching between the transmitter and antenna. A structure which reduces the effects of resonance behavior will provide improved impedance matching. A typical definition of the useful bandwidth of an antenna would be those spectrum regions where the voltage-standing-wave-ratio is three-to-one or less. Some critical applications might call for two-to-one and some non-critical applications might well tolerate five-to-one or six-to-one values of voltage-standing-wave-ratio. Absolute perfection of a one-to-one voltage-standing-wave-ratio is rarely necessary and generally achieved only with antennas designed to operate on a single discrete frequency.
There are numerous versions of prior-art antenna configurations with wideband, multiple-band or all-frequency performance in the high-frequency spectrum.
Techniques which are employed include the use of manual tuning or servomechanism tuning so as to regain resonance and proper impedance matching. While effective, this technique is unwieldy or expensive.
Also included are multiple antennas, connected to a common feed point, each accepting radio frequency energy at its own resonant frequency. When the number of bands becomes high, this configuration becomes difficult to adjust and suffers from appearance and practical construction problems.
Further techniques are the use of tuned circuit elements or transmission line segments to act as bandstop networks which electronically disconnect portions of an antenna structure so that the remainder is resonant. These are commonly called "traps" and are effective for two or three band performance. Since the components employed are subjected to high voltage, care is required in the basic design to provide adequately rated components and humidity-resistant construction.
Prior art also includes the use of resistive termination elements, which, by sacrificing efficiency, can reduce reflected waves on the antenna and improve its behavior across frequency spans.
Recent technology includes the use of structures which show repetitive ratio construction with each fixed frequency change percentage. Known as "log-periodic" structures, these are probably the most successful in providing wide bandwidth performance. Such structures are generally neither simple nor inexpensive when implemented in the high-frequency region of the spectrum.
A review of available techniques shows there remains a need for a relatively simple, inexpensive structure which provides efficient radiation and moderately good impedance matching characteristics over a number of bands.